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If Smart Is the Norm, Stupidity Gets More Interesting

Few of us are as smart as we’d like to be. You’re sharper than Jim (maybe) but dull next to Jane. Human intelligence varies. And this matters, because smarter people generally earn more money, enjoy better health, raise smarter children, feel happier and, just to rub it in, live longer as well.

But where does intelligence come from? How is it built? Researchers have tried hard to find the answer in our genes. With the rise of inexpensive genome sequencing, they’ve analyzed the genomes of thousands of people, looking for gene variants that clearly affect intelligence, and have found a grand total of two.

One determines the risk of Alzheimer’s and affects I.Q. only late in life; the other seems to build a bigger brain, but on average it raises I.Q. by all of 1.29 points.

Other genetic factors may be at work: A report last year concluded that several hundred gene variants taken together seemed to account for 40 to 50 percent of the differences in intelligence among the 3,500 subjects in the study. But the authors couldn’t tell which of these genes created any significant effect. And when they tried to use the genes to predict differences in intelligence, they could account for only 1 percent of the differences in I.Q.

But is the genetic cup really empty, or are we just looking for the wrong stuff? Kevin Mitchell, a developmental neurogeneticist at Trinity College Dublin, thinks the latter. In an essay he published in July on his blog, Wiring the Brain, Dr. Mitchell proposed that instead of thinking about the genetics of intelligence, we should be trying to parse “the genetics of stupidity,” as his title put it. We should look not for genetic dynamics that build intelligence but for those that erode it.

The premise for this argument is that once natural selection generated the set of genes that build our big, smart human brains, those genes became “fixed” in the human population; virtually everyone receives the same set, and precious few variants affect intelligence. This could account for the researchers’ failure to find many variants of measurable effect.

But in some other genetic realms we do differ widely, for example, mutational load — the number of mutations we carry. This tends to run in families, which means some of us generate and retain more mutations than others do. Among our 23,000 genes, you may carry 500 mutations while I carry 1,000.

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Lars Leetaru

Most mutations have no effect. But those that do are more likely to bring harm than good, Dr. Mitchell said in an interview, because “there are simply many more ways of screwing something up than of improving it.”

Open the hood of a smooth-running car and randomly turn a few screws, and you’ll almost certainly make the engine run worse than before. Likewise, mutations that change the brain’s normal development or operation will probably slow it down. Smart Jane may be less a custom-built, high-performance model than a standard version pulling a smaller mutational load.

We also inherit — through genes yet to be identified, of course — a trait known as developmental stability. This is essentially the accuracy with which the genetic blueprint is built. Developmental stability keeps the project on track. It reveals itself most obviously in physical symmetry. The two sides of our bodies and brains are constructed separately but from the same 23,000-gene blueprint. If you have high developmental stability, you’ll turn out highly symmetrical. Your feet will be the same shoe size, and the two sides of your face will be identical.

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If you’re less developmentally stable, you’ll have feet up to a half-size different and a face that’s like two faces fused together. Doubt me? Take a digital image of your face and split it down the middle. Then make a mirror-image copy of each half and attach it to its original. In the two faces you’ve just made — one your mirrored left side, the other your right — you’ll behold your own developmental stability, or lack thereof.

Both those faces might be better-looking than you are, for we generally find symmetrical faces more attractive. It also happens that symmetry and intelligence tend to run together, because both run with developmental stability. We may find symmetrical faces attractive because they imply the steadiness of genetic development, which creates valuable assets for choosing a mate, like better general fitness and, of course, intelligence — or as Dr. Mitchell might put it, a relative lack of stupidity.

These ideas don’t strike geneticists as radical or contrary. Leonid Kruglyak, a Princeton geneticist who studies yeast and roundworms, noted in an e-mail that geneticists had long recognized that mutations could “throw sand in the gears of the brain” and that complex traits arose in complicated ways. “Talking about ‘a gene for a trait’ is a shorthand at best,” he wrote, “and a well-known fallacy at worst.”

Dr. Mitchell agreed. “This isn’t a brand-new idea,” he said. “But it’s not one that has been generally adhered to in intelligence studies.”

Not brand-new, perhaps. But it’s this kind of “inversion of thought” (as Janet Kwasniak, a retired biologist, put it on her neuroscience blog, Thoughts on Thoughts) that can often spark new approaches to intractable problems.

Dr. Jay Giedd, who studies brain development at the National Institutes of Health, has done research suggesting that the brain blooms through many small arcs of development that make it responsive to experience — and vulnerable to error. At first, he said in an e-mail, he was skeptical of Dr. Mitchell’s idea. Then he discussed it with colleagues at a neuroscience meeting.

“My initial thought was that it would be easy to sink the argument,” Dr. Giedd said. But the more they discussed it, the more sense it made. “Everybody I ran it by seemed to feel the logic is sound.”

Correction: October 25, 2012

An article on Tuesday about efforts to understand the genetic factors that build or erode intelligence misidentified one of the research subjects of the Princeton geneticist Leonid Kruglyak. Dr. Kruglyak studies yeast and roundworms, not flatworms.

David Dobbs is the author of the coming book “Orchids and Dandelions,” which will explore the genetic and cultural roots of temperament.

A version of this article appears in print on October 23, 2012, on Page D5 of the New York edition with the headline: If Smart Is the Norm, Stupidity Gets More Interesting. Order Reprints|Today's Paper|Subscribe